Method for producing pig iron in a shaft furnace

ABSTRACT

A method of producing pig iron in a shaft furnace is provided. The shaft furnace is charged in an upper region with raw materials which fall within the shaft furnace under the influence of gravity. A portion of the raw materials is melted and/or partly reduced under the action of the atmosphere that exists within the shaft furnace. A hot gas stream which is introduced in a lower region of the shaft furnace flows through and influences the atmosphere that exists within the shaft furnace in terms of chemical composition and temperature. A cold gas stream is fed to a heat exchanger in which the cold gas stream is heated to a temperature higher than 700° C. to give a hot gas stream. The cold gas stream comprises a CO 2  component of at least 5% by volume. The cold gas stream may contain, air and/or pure oxygen as residual component.

The invention relates to a method of producing pig iron in a shaft furnace which is charged in an upper region of the shaft furnace with raw materials which fall within the shaft furnace under the influence of gravity, wherein a portion of the raw materials is melted and/or at least partly reduced under the action of the atmosphere that exists within the shaft furnace, and a hot gas stream which is introduced in a lower region of the shaft furnace flows through, especially in countercurrent, and influences the atmosphere that exists within the shaft furnace in terms of chemical composition and/or temperature, wherein a cold gas stream, especially upstream of the shaft furnace process, is fed to at least one heat exchanger in which the cold gas stream is heated to a temperature higher than 700° C. to give a hot gas stream.

The production of pig iron in shaft furnaces, for example in blast furnaces, that are operated in a sustained manner is globally the most common and standard method, by which far more than 80% of the global demand for pig iron is produced. Conventionally, in the reduction method, the upper region of the shaft furnace is charged via what is called the furnace top with raw materials, known as the burden, comprising iron ore and optionally lime, coke and/or coal and, if required, further additives or other oxidic or metallic starting materials/feedstocks. By virtue of gravity and batchwise charging, especially as a result of discontinuous tapping and hence removal of the liquid substances, the raw materials drop down. By virtue of the chemical composition of the atmosphere (reducing condition), and as a result of the temperature increasing in the direction of the lower region, the iron ore is reduced. In the lower region of the shaft furnace, especially in the region of introduction of hot gas streams, called hot blast, and optionally further additives, such as carbon and/or oxygen, the temperature is such as to cause the iron ore or reduced iron to melt, and different liquid phases are established in what is called the hearth region, which collects at different levels into liquid iron and a molten slag that covers the liquid iron on account of its lower density. In particular, the liquid phases can each be drawn off via different discharges/tapping points, and these may be sent to further processes.

Conventionally, the hot blast, also called hot air, is ambient air which is sucked in as a cold gas stream, called cold blast, via a starting bar, compressed to a defined pressure and fed to at least one blast heater in which the compressed ambient air (cold blast) is heated to a temperature of at least 700° C., and this is then introduced or blown into the blast furnace via what are called blast tuyeres in the lower region of the shaft furnace. Heat exchangers in the form of blast heaters, also called Cowper stoves, are alternately flooded with a hot gas, where the hot gas remains in the blast heater until a defined temperature is attained within the blast heater, then the hot gas is removed and flooded with compressed ambient air (cold blast) and remains in the blast heater until a predefined temperature, generally between 700 and 1400° C., has been attained, such that the hot ambient air is then drawn off as hot blast (hot blast stream) and fed to the blast furnace. An economically viable method of charging the blast heater with hot gas that has become established is the utilization of the waste heat stream which is drawn off as what is called top gas from the furnace top, which is reincinerated with further gases (air/natural gas) in the blast heater. In order to maintain sustained operation and depending on the size of the blast furnace, at least two, especially at least three, blast heaters are connected in parallel and can be appropriately switched on and off, such that, for example, one of the blast heaters is being flooded with hot gas and utilized for heating of the blast furnace with release of heat, another blast heater is being flooded with cold blast and heated with absorption of heat to give hot blast, and a third blast heater is operational and is supplying the blast furnace with hot blast. In principle, it is also possible to use two blast heaters that are flooded in alternating operation.

The carbon-based production of pig iron gives rise to an enormous CO₂ output which pollutes the environment in that increasing the CO₂ boosts the greenhouse effect in the atmosphere and hence adversely affects the climate. For many years, operators of shaft furnaces have been contemplating how this output can be reduced. Exacerbated by national/international stipulations associated with CO₂ constraints or certificates, the operators of these plants are being forced to take measures that still permit operation of these plants under particular prerequisites. Economic operation should also still be ensured.

The prior art describes numerous approaches for achieving a positive and reducing effect on CO₂ output by means of controlled addition of media (solid, liquid and gaseous) to a shaft furnace. By way of example, publications CN 101871026 A1 and WO 2019/057930 A1 are mentioned.

As well as CO₂, the conventional shaft furnace process also results in other, in particular “harmful”, gas compounds and liquids within a flow of matter that can have adverse effects not just in the shaft furnace process but also in downstream applications. The use of air (cold gas stream, cold blast), which consists of nitrogen to an extent of about 79% by volume and is used as hot gas stream or hot blast for blowing into the blast furnace, depending on temperature and pressure, contributes to the formation of nitrogen oxide emissions (NOx), hydrogen cyanide (HCN) and potassium cyanide compounds in the atmosphere of the shaft furnace and in the top gas, optionally in the blast heater. Nitrogen is additionally inert, and its presence does not result in optimization in the process. Instead, nitrogen contributes, for example, to a reduced calorific value in the top gas and affects the Wobbe index (WI). Efficiency in the blast furnace process (including up- and downstream) is nonoptimal as a result of the presence of nitrogen. Furthermore, the nitrogen compounds formed, especially hydrogen cyanide (HCN) and potassium cyanide compounds, lead to elevated nitrogen oxide emissions (NOx) on utilization, especially in the combustion of the top gas and the gas mixtures formed therefrom. A high level of economic detriment (deNOx plants) is necessary, especially in the power plant or sintering plant, in order to be able to comply with relevant offgas limits.

It is therefore an object of the invention to specify a method of producing pig iron in a blast furnace, with which an optimal process can be provided, and nitrogen oxide emissions and other nitrogen compounds, such as hydrogen cyanide or potassium cyanide compounds, can be reduced or essentially prevented.

The object is achieved by the features of claim 1.

The inventors have found that air (cold gas stream, cold blast) which is sucked in in the existing conventional process can be partly or fully replaced by supplying CO₂. According to the invention, the cold gas stream, before being introduced into the at least one heat exchanger, comprises a CO₂ component of at least 5% by volume, wherein the cold gas stream, as well as impurities, may contain air and/or pure oxygen as residual component. The cold gas stream may especially comprise at least 10% by volume, preferably at least 20% by volume, more preferably at least 30% by volume, especially preferably at least 40% by volume, of CO₂, in order to reduce or to partly or fully replace the air component in the cold gas stream.

Impurities, especially unavoidable impurities, in the context of the invention are components or accompanying elements in the composition of the cold gas stream/hot gas stream, which may be present up to 2.0% by volume, especially up to 1.5% by volume, preferably up to 1.0% by volume, but do not make any significant contribution or are of no consequence and hence do not have any influence on the process. Using the example of air, the main components are oxygen and nitrogen, and impurities include noble gas (argon) and carbon dioxide, adding up to about 1% by volume.

Depending on the shaft furnace and/or mode of operation of the shaft furnace, proportions of the air in the cold gas stream may be effectively substituted or displaced by CO₂, such that less nitrogen is fed in or circulated in the volume flow or in the flow of matter throughout the process. As a result, firstly, a reduction in NOx, HCN and potassium cyanide compounds is ensured and, secondly, use of CO₂ in the cold gas stream has a positive effect on the CO₂ balance over the entire process. CO₂ has a higher specific heat capacity than air, which is about 26% higher, such that the efficiency of the heat exchanger or of the blast heater (Cowper stove) can be increased, since the same volume flow rate can achieve a higher calorific power density. Moreover, the viscosity (kinematic, dynamic) of CO₂ is lower compared to air, which can have an advantageous effect in the heat exchanger/blast heater and in the shaft furnace, including up- and downstream.

The CO₂ heated to the temperatures that are customary nowadays in the heat exchanger (blast heater) is unstable within this temperature range, >700 to 1400° C., meaning that it would break down to CO after being blown in in the lower region of the blast furnace in what is called the raceway in contact with a (substitute) reducing agent with consumption of tangible heat. In the case of reaction with carbon, for example, about 172 kJ/mol is consumed. In the case of reaction with hydrogen, for example, only about 30.9 kJ/mol. This heat consumption is essentially covered by the heat released simultaneously in the combustion with oxygen, such that there can still be a considerable energy surplus in net terms, depending on the mixing ratio. Thus, a metallurgically effective (meaning that it is available as a reducing agent) optimized hot gas stream (hot blast) is available from the start in the raceway, which can additionally increase the efficiency of the blast furnace.

Carbon is introduced directly with the CO₂ in the hot gas stream (hot blast) via the blast tuyeres, such that it is possible to reduce the amount of raw material, especially the feed of coke, for example, but also the additional injection of carbon, compared to conventional operation.

Further advantageous configurations and developments will be apparent from the description that follows. One or more features from the claims, the description and the drawings may also be combined with one or more other features thereof to give further configurations of the invention. It is also possible for one or more features from the independent claims to be linked by one or more other features.

In one configuration of the method of the invention, the cold gas stream (cold blast) contains CO₂, air and optionally pure oxygen in addition to impurities, where the proportion of air is limited to not more than 50% by volume, especially to not more than 40% by volume, preferably to not more than 30% by volume, more preferably to not more than 20% by volume, especially preferably to not more than 10% by volume. The partial use of air has the advantage, for example, that the cold gas stream or hot gas stream has a certain “moisture content” in the form of water or water vapor in the stream of matter (hot blast), which depends on the proportion of air in the cold/hot gas stream and on ambient conditions, such that it is either sufficient or is adjustable once again in a controlled manner by the introduction of, for example, water vapor into the hot gas stream prior to introduction of the hot gas stream (hot blast) into the shaft furnace. The “blast moisture content” may be advantageous for a calm and uniform mode of operation in the shaft furnace. The regulation of the blast moisture content can control the combustion temperature in the raceway, called the RAFT, via the endothermic properties. At the same time, blast moisture content can also affect the hydrogen content in the top gas.

The pure oxygen optionally introduced into the cold gas stream may be brought to temperature, by using it as oxidizing agent for release of heat and as reducing agent, especially carbon monoxide, in order thus to increase the efficiency by increasing the power density. This effect is partly based on the fact that the ratio of oxygen to nitrogen is increased. When CO₂ is used, it is possible to partly or completely dispense with a further separate injection of oxygen. Alternatively, the optional pure oxygen can also first be introduced into the hot gas stream (hot blast) before the introduction of the hot gas stream into the shaft furnace, in order to prevent any possible reaction with air or the nitrogen in the air to give NOx, HCN, especially in the heat exchanger. What is meant by “optionally” in this connection is that no pure oxygen is supplied either, either to the cold gas stream or the hot gas stream.

In an alternative configuration of the method of the invention, the cold gas stream (cold blast) contains CO₂ and optionally pure oxygen in addition to impurities, where the proportion of CO₂ is at least 70% by volume, especially at least 75% by volume, preferably at least 80% by volume, more preferably at least 85% by volume, especially preferably at least 90% by volume. The cold gas stream is effectively air- or nitrogen-free, such that no NOx emissions can be released in the shaft furnace at least by the hot gas stream, and the top gas is thus also free of nitrogen and nitrogen compounds, such that the top gas has a better calorific value and better emissions values compared to the conventional derived top gas. On account of the essentially nitrogen-free top gas, the use thereof is suitable not just for increasing the efficiency of the heat exchanger (blast heater), but also for direct separation of CO₂, or downstream separation of CO₂ especially in what are called oxyfuel processes.

The pure oxygen optionally introduced into the cold gas stream can be brought to temperature, wherein it oxidizing agent is used for release of heat and the formation of reducing agent, especially carbon monoxide (CO), in order thus to increase the efficiency by increasing the power density. This effect is partly based on the fact that the ratio of oxygen to nitrogen is increased. When CO₂ is used, it is possible to partly or completely dispense with a further separate injection of oxygen. Alternatively, the optional pure oxygen may also be introduced only into the hot gas stream before introduction of the hot gas stream into the shaft furnace. What is meant by “optionally” in this connection is that no pure oxygen is supplied either, either to the cold or to the hot gas stream. The cold gas stream, and possibly also the hot gas stream, may in the specific case consist solely of CO₂ together with impurities.

In a preferred configuration of the method of the invention, CO₂ for the cold gas stream is provided from a CO₂ separation, which is either separated out of an offgas combusted in the heat exchanger or blast heater or can be produced or deposited from other processes which especially in the immediate proximity in the metallurgy plant. Other processes are, for example, the use/separation of CO₂ from a direct reduction (DR), which may be coupled to the shaft furnace process (integrated metallurgy plant). Alternatively, it is also conceivable to provide CO₂ as a pure industrial gas or else with low demands on purity.

In one configuration of the method of the invention, hydrogen is additionally introduced in the lower region of the shaft furnace. Hydrogen as what is called a replacement reducing agent in conjunction with the CO₂ introduced from the hot gas stream (hot blast) can contribute to lowering the demand for coal and coke and hence to an increase in economic viability. The use of hydrogen may, for example, be 0.005% to 0.1%, especially 0.01% to 0.08%, preferably 0.015% to 0.07%, per tonne of pig iron produced.

In one configuration of the method of the invention, pure oxygen is additionally introduced in the lower region of the shaft furnace. If the optional pure oxygen introduced via the hot gas stream (hot blast) is insufficient for combustion or no additional pure oxygen is introduced with the hot gas stream, separate introduction of oxygen may be required in order to ensure the necessary energy for operation of the shaft furnace. The use of additional oxygen may, for example, be 0.1% to 13% per tonne of pig iron produced. In the case of operation without additional oxygen, a half mole of oxygen is released from the hot CO₂-containing gas stream per mole of CO₂.

In one configuration of the method of the invention, carbon is additionally introduced in the lower region of the shaft furnace. If the carbon from the CO₂ of the hot gas stream introduced via the hot gas stream (hot blast) and/or the input of coal/coke via the top is insufficient, separate introduction of carbon may be required to generate heat input into the shaft furnace, in order that the temperature level that takes place for the reduction reactions in the furnace is attained. Target temperatures in the raceway are in the range between 1800° C. and 2500° C.

In one configuration of the method of the invention, the hydrogen and the oxygen are produced and provided from a (water or chloralkali) electrolysis. Also conceivable are other hydrogen sources, especially the hydrogen present in the blast furnace gas or the coking furnace gas mixture per se. In addition, a high-temperature electrolysis is also conceivable, since this can be integrated efficiently with the wide variety of different heat sources in an integrated metallurgy plant. The oxygen may also come from other sources, for example air fractionation plants.

In one configuration of the method of the invention, if air is to be supplied to the cold gas stream (cold blast), at least the air component of the cold gas stream is compressed to a pressure above ambient pressure before being combined with the other components, before being combined with the other components and before the introduction of the cold gas stream into the at least one heat exchanger. Introduction of compressed air into the heat exchanger (blast heater) corresponds to the conventional course of action. The CO₂ and/or optionally pure oxygen components may already be provided with a pressure above ambient pressure as a result of the process, such that this/these component(s) is/are supplied to the cold gas stream after compression, and hence the throughput through the compressor and thus the operating costs of the compressor can be reduced.

If the components or component of the cold gas stream are provided in unpressurized form or at the level of ambient pressure, the cold gas stream is compressed, especially completely, to a pressure above ambient pressure, before the cold gas stream is introduced into the at least one heat exchanger (blast heater).

There follows a detailed elucidation of specific configurations of the invention with reference to the drawing. The drawing and accompanying description of the resulting features should not be read as being restricted to the respective configurations, but instead serve for illustration of exemplary configuration. In addition, the respective features may be utilized together with one another or else together with features of the above description for further possible developments and improvements of the invention, specifically in the case of additional configurations that are not shown. Identical parts are always given the same reference numerals.

The drawing shows:

FIG. 1 ) a schematic of a blast furnace with an upstream blast heater and corresponding streams of matter in a conventional mode of operation and

FIG. 2 ) a schematic of a blast furnace with an upstream blast heater and corresponding streams of matter in an inventive mode of operation.

FIG. 1 shows a schematic of a conventional blast furnace with an upstream blast heater. Although only one blast heater (heat exchanger) is shown in a symbolic manner, there are in principle at least two, especially at least three, blast heaters (heat exchangers) disposed in the periphery of the shaft furnace/blast furnace. The mode of operation of the heat exchanger(s) (blast heater(s)) is prior art. Conventionally, air is sucked in from the environment, guided through compressors (not shown) and compressed, and introduced as cold gas stream (cold blast) into at least one of the heat exchanger(s) (blast heater(s)) which is already at an appropriate temperature. The blast heater is flooded with cold blast, and the heat stored in the blast heater is transferred to the compressed cold blast and, on attainment of a predefined temperature, generally between 700° C. and 1400° C., fed as hot gas stream (hot blast) to the blast tuyeres (tuyeres, nozzles) of a shaft furnace or blast furnace in which pig iron has been produced. In the upper region of the shaft furnace, raw materials required for production of pig iron are charged via the furnace top. Under the influence of gravity, the raw materials drop down within the shaft furnace, with melting and/or at least partial reduction of a portion of the raw materials under the action of the atmosphere that exists within the shaft furnace. In the lower region of the shaft furnace, a hot gas stream (hot blast) is introduced, which flows through the atmosphere within the shaft furnace in countercurrent and affects the chemical composition and temperature thereof. In addition, and depending on the mode of operation, it is possible to introduce carbon (carbon-based additives) and/or oxygen in the lower region of the shaft furnace separately from the hot gas stream. The blast furnace process and the mode of operation thereof are also prior art. Conventionally, air with about 79% by volume of nitrogen and about 21% by volume is introduced into the shaft furnace as cold gas stream (cold blast) or hot gas stream (hot blast). Coke is used as fuel and carbon carrier for the reduction of the iron ore, the primary material from which pig iron is to be produced in the blast furnace, and this coke, which, like the iron ore, is introduced into the shaft furnace via the top as the burden in a layered or mixed manner in each case, and if required additionally coal dust, which is additionally injected especially via the blast tuyeres. Alternatively, and depending on the plant design, it is also possible to use heating oil, natural gas, coking furnace gas, plastic or hydrogen, for example, as replacement reducing agent, which are injected via specific devices. The top gas exits at the top at about 140° C. to 250° C., and between 1500-1850 standard cubic meters (m³ (STP)) of top gas may be obtained per tonne of pig iron in a normal mode of operation. Some of the top gas or all of the top gas serves as fuel for the blast heater, which is mixed with further gases, for example natural gas and air, and recombusted. A composition of the top gas measured in normal operation contains, in % by volume: CO at 21%, CO₂ at 21%, H₂ at 2% and N₂ at 56%, of which there may be HCN at 0.0025% to 1.2% and NOx at 0.001% to 0.15%.

FIG. 2 shows a schematic of the same conventional shaft furnace (blast furnace) with an upstream heat exchanger (blast heater), but with the difference that, in accordance with the invention, CO₂ is used wholly or partly as cold gas stream (cold blast). Pure oxygen is given an * in FIG. 2 , which is supposed to mean that oxygen is introduced either into the cold gas stream and/or into the hot gas stream before the hot gas stream (hot blast) is introduced into the shaft furnace. Alternatively, it is also possible for only CO₂ together with impurities to be used as cold/hot gas stream, or for CO₂ in conjunction with air up to a maximum of 50% by volume and/or with pure oxygen up to 30% by volume (not shown here). The inventive example in FIG. 2 shows that, with virtually 100% by volume of CO₂, firstly, the CO₂ balance of the overall process is positive and, secondly, the level of nitrogen in the process can be reduced, or it is absent, such that also only reduced to zero NOx emissions and reduced to zero potassium cyanide compounds or hydrogen cyanide are obtained within the process by comparison with the conventional mode of operation. In one experiment, air was replaced completely by CO₂ in the cold gas stream (cold blast), with separation of CO₂ from a direct reduction process (alternatively oxyfuel process) and provision with a pressure of 6 bar, such that there was no longer any need to use the conventionally present (blast) compressor and it was thus possible to save power for this piece of equipment. In the heat exchanger (blast heater), the CO₂ was heated to 1200° C. and injected into the blast furnace as hot gas stream (hot blast). In addition, hydrogen was injected at up to 1000 m³ (STP)/h, especially per blast tuyere (tuyere), where the blast furnace may have multiple (blast) tuyeres, in which case it is especially possible, in the case of reduction of hydrogen, additionally to separately inject carbon, for example as coal powder, and/or oxygen. Via the top, especially between 250 and 400 kg of coke per tonne of pig iron produced is fed in. Up to 12 000 m³ (STP) of top gas was obtained per tonne of pig iron, for which a composition was ascertained in % by volume: CO₂ at 47%, CO at 38% and H₂ at 15%. Contamination with nitrogen or “harmful” NOx, potassium cyanide compounds or hydrogen cyanide was not present in the top gas, and so it had an improved calorific value and better emission characteristics. It was possible to reduce the use of further gases for the post-combustion in the heat exchanger (blast heater), dispensing in this case with air and using pure oxygen for nitrogen-free combustion. The offgas/combustion gas led off from the blast heater after combustion was of excellent suitability for recycling since, because there are few troublesome components in the offgas, the CO₂ can be fed back to the (blast furnace) process relatively easily and effectively as recycled CO₂.

The invention is also implementable with proportions of air and/or pure oxygen in the cold gas stream (cold blast), since at least 5% by volume, especially at least 10% by volume, preferably at least 20% by volume, more preferably at least 30% by volume, especially preferably at least 40% by volume of CO₂ in the cold gas stream (cold blast) and hence partial to complete replacement of the air can lead to a reduction in NOx emissions caused by nitrogen, and this can, for example, also increase/improve the efficiency of the (overall) process.

The invention is applicable to any type of shaft furnace, i.e. not just restricted to blast furnaces, but is also implementable in cupola furnaces, primary energy furnaces etc. that work by the principle of action described. 

1-10 (canceled)
 11. A method of producing pig iron in a shaft furnace which is charged in an upper region of the shaft furnace with raw materials which fall within the shaft furnace under the influence of gravity, wherein a portion of the raw materials is at least one of melted and at least partly reduced under the action of the atmosphere that exists within the shaft furnace, and a hot gas stream which is introduced in a lower region of the shaft furnace flows through and influences the atmosphere that exists within the shaft furnace in terms of chemical composition and temperature, wherein a cold gas stream is fed to at least one heat exchanger in which the cold gas stream is heated to a temperature higher than 700° C. to give a hot gas stream, wherein the cold gas stream, before being introduced into the at least one heat exchanger, comprises a CO₂ component of at least 5% by volume, wherein the cold gas stream contains, aside from impurities, at least one of air and pure oxygen as residual component.
 12. The method as claimed in claim 11, wherein the cold gas stream contains CO₂, air and pure oxygen in addition to impurities, where the proportion of air is limited to not more than 50% by volume.
 13. The method as claimed in claim 11, wherein the cold gas stream contains CO₂ and pure oxygen in addition to impurities, wherein the proportion of CO₂ is at least 70% by volume.
 14. The method as claimed in claim 12, wherein the CO₂ is provided from a CO₂ separation.
 15. The method as claimed in claim 14, wherein hydrogen is additionally introduced in the lower region of the shaft furnace.
 16. The method as claimed in claim 15, wherein pure oxygen is additionally introduced in the lower region of the shaft furnace.
 17. The method as claimed in claim 16, wherein carbon is additionally introduced in the lower region of the shaft furnace.
 18. The method as claimed in claim 17, wherein the hydrogen is produced and provided from electrolysis, and the pure oxygen from an air fractionation plant.
 19. The method as claimed in claim 12, wherein at least the air component of the cold gas stream, before being combined with the other components, is compressed to a pressure above the ambient pressure before being combined with the other components and before the cold gas stream is introduced into the at least one heat exchanger.
 20. The method as claimed in claim 12, wherein the cold gas stream is compressed to a pressure above the ambient pressure before the cold gas stream is introduced into the at least one heat exchanger. 